1. Technical Field
The present invention relates to a rubber composition and a pneumatic tire using the rubber composition.
2. Related Art
Conventionally, a wax is added to a rubber composition constituting, for example, a side wall part of a pneumatic tire in order to improve ozone resistance. The wax blooms on the surface of a rubber after vulcanization to form a coating film of a wax on the surface of a rubber, thereby cutting off ozone and protecting the rubber. On the other hand, a resin as a tackifier is sometimes added to a rubber composition in order to improve adhesion to rolls in a kneading step. Rubber compositions having a wax and a tackifier added thereto are known, and various rubber compositions are proposed (for example, see JP-A-2010-053249, JP-A-2013-082884 and JP-A-2000-086824).
The present inventors found the following new problem in a rubber composition having both a wax and a tackifier added thereto. That is, it was ascertained that where a large amount of a tackifier is added in order to improve adhesion to rolls, the tackifier moves to the surface of a rubber prior to a wax and cures thereon, and the cured resin film cracks, and additionally, the crack propagates on the surface of a rubber, thereby cracks occur on the surface of a rubber. It was further ascertained that the tackifier moved to the rubber surface disturbs blooming of a wax, thereby accelerating occurrence of cracks by ultraviolet rays.
The present embodiment is made for solving the new problem and has an object to provide a rubber composition that enables a wax to bloom prior to that a tackifier moves to the rubber surface and can suppress the occurrence of cracks by ultraviolet rays while improving adhesion to rolls, even in the case where a large amount of the tackifier has been added.